High-pressure discharge lamp electrode having a dendritic surface layer thereon

ABSTRACT

In the case of an electrode ( 5 ) for a high-pressure discharge lamp, at least a part of the surface of the electrode is covered with a dendritic layer ( 13 ) made from high-melting-point metal. A substantially longer service life is achieved thereby.

TECHNICAL FIELD

The invention proceeds from an electrode for a high-pressure dischargelamp in accordance with the preamble of claim 1. At issue, inparticular, are mercury short-arc lamps, in particular for thesemiconductor industry. There, they are frequently used inphotolithographic processes for exposing wafers or other substrates. Afurther preferred field of application is inert gas high-pressuredischarge lamps, in particular xenon high-pressure discharge lamps.Application for metal halide lamps is also possible.

PRIOR ART

Already known from publication EP-A 791 950 is a high-pressure dischargelamp in which the anode is provided outside its tip with a sintered-onlayer made from fine-grained tungsten. The surface of the anode isenlarged thereby. The temperature of the anode can thus be loweredduring operation, and the bulb blackening can be reduced. The emissivityof such a layer is approximately 0.5.

DE-A 11 82 743 discloses the use of a layer which raises the emissivityand is made from sintered-on tungsten or TaC. The layer is applied tothe anode in this case by slurrying a suspension of butyl acetate withcellulose binder and the corresponding metal powder. The sinteringprocess is performed under a vacuum at temperatures above 1800° C.Additional cooling can be achieved by using cooling channels 1-3 mmdeep.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide an electrode for ahigh-pressure discharge lamp in accordance with the preamble of claim 1which has a very long service life.

This object is achieved by means of the characterizing features ofclaim 1. Particularly advantageous refinements are to be found in thedependent claims.

The coating according to the invention of the surface of an electrode issuitable as an extremely effective mechanism for cooling the electrode(by thermal radiation). The point is that the higher the emissioncoefficient the cooler the electrode becomes. Consequently, the tungstenevaporation from the electrode, and thus the bulb blackening can bereduced. Because of the exponential increase in the tungsten evaporationrate with the temperature of the electrode, even a comparatively slightdrop in temperature leads to a substantial reduction in the bulbblackening.

In photolithography, in particular, it is required of the lamp that thereduction in the luminous flux should be as slight as possible in thecourse of the lamp operation. An alternative is the desire for aluminous flux which is as high as possible, so that it is possible toachieve a very short exposure time of the substrate. Consequently, alengthening of the service life can be achieved, on the one hand.Alternatively, design possibilities are opened up for achieving a higherinitial intensity in conjunction with constant maintenance. Thedimensions of the electrodes can also possibly be reduced.

The reason for the reduction in the luminous flux is that the electrodematerial (tungsten being used as a rule) can melt and evaporate in thedischarge arc in the case of a high power density. The anode, inparticular, is heated up strongly by the impact of the electrons.Tungsten evaporating from the anode is deposited on the bulb and leadsto bulb blackening which reduces the luminous flux of the lamp. However,the invention can also be applied in the case of highly loaded cathodes.

The anode temperature depends in this case essentially on the poweremitted by it. If the anode is regarded as a Planckian radiator, theemitted power per area (L) is described by the Stefan-Boltzmann law:

L=ε×σ×T ⁴

Here, σ=5.67×10⁻⁸ W m⁻² K⁻⁴ is the Stefan-Boltzmann constant; theemission coefficient ε describes the deviation of a thermal radiator(0<ε<1) from an ideal blackbody radiator. (ε=1). T is the temperature inK.

In the present invention, the coating of the anode with a dendriticmetal or a metal compound increases the emission coefficient fromapproximately 0.3 (pure tungsten) to values above 0.6 (in the case of atemperature of at least 1000° C.) . Values of over 0.8 are even reachedfor the first time in lamp construction. The dendritic structure isunderstood here as a multiplicity of needle-shaped, radiation-reflectingoutgrowths on the otherwise smooth surface. These outgrowths are locatednext to one another at a spacing of a few nanometers to more than ahundred micrometers, preferably at a mean spacing of at least 300 nm. Astructure in which the depth of the valley between two neighbouringneedle-shaped peaks is at least 30% of the spacing of these peaks fromone another has proved to be particularly suitable. The dendritic layercan be produced, in principle, from high-melting-point metals.Particularly suitable are rhenium, tungsten, molybdenum and tantalum ortheir carbides and/or nitrides. Carbides or nitrides of hafnium orzirconium are also suitable. In addition, a normal coating made from ahigh-melting-point metal can be applied between the core of tungsten andthe dendritic layer.

A rhenium layer is particularly suitable, since a dendritic structurecan be produced particularly effectively thereby. Its emissioncoefficient ε is approximately 0.9. Consequently, for a prescribedemitted power L it is possible in the case of an anode coated withdendritic rhenium to reduce the temperature by up to 200 K whenoperating the anode, by comparison with an uncoated anode, or one coatedwith sintered-on tungsten or TaC. The suitability of the rhenium layerfor lamp construction is astonishing to the extent that the vapourpressure of the rhenium is higher by a factor of approximately 75 bycomparison with tungsten. This point of view plays no role in the caseof a rotary anode operated in a vacuum, since the vapour-depositedmaterial condenses at cold spots. However, in lamp construction theintense deposition would lead to blackening, and thus to reduced servicelife. Because of the substantial temperature drop, this disadvantageouseffect is more than balanced out, however.

This greatly improved anode cooling furthermore greatly reduces theevaporation of the regular electrode material (tungsten) from thedeposition surface of the anode facing the discharge. As a consequencethereof, the lamp is distinguished overall in the case of identicallight data by a substantially diminished reduction in radiation in thecourse of the service life.

The front region of the anode is preferably hemispherical or conicallytapered. Particularly suitable is a conical frustum with a planedeposition surface for the discharge (called the anode plateau in thefollowing text).

Alternatively, the invention can provide anodes with smaller dimensionsin conjunction with an unchanged service life response and the sameoperating temperature. The smaller dimensioning reduces the shading ofthe discharge arc by the electrodes, as a result of which the luminousflux of the lamp is increased in conjunction with the same service liferesponse.

For example, it is possible by means of chemical gas phase epitaxy (alsoknown in technical language as CVD (Chemical Vapor Deposition)) to applyto the surface of the anode a metal layer, approximately 10 to 40 μmthick, with a dendritic surface morphology. It is characterized byneedle-shaped crystallites whose mutual spacing is typicallyapproximately 10-30 μm. The needle-shaped crystallites are positionedapproximately perpendicularly on the surface, with the result thatincident radiation is virtually completely absorbed by multiplereflection between the lateral surfaces of neighbouring crystals. As aresult, such a layer has a high absorptivity and is black. In accordancewith the high absorptivity, it has a high emission coefficient of up toε=0.9. The production of such layers is described in U.S. Pat. No.3,982,148 in connection with an application in rotary anodes for X-raytubes. Reference is expressly made to this publication. The CVDtechnique is particularly suitable as a method of production for thislayer. However, other techniques for the production of thin,high-melting-point, metal layers such as, for example, sputtering (oftendesignated in technical language as PVD (Physical Vapor Deposition)) orlaser ablation also come into consideration.

The increase in the emission coefficient to values of up toapproximately 0.9 can lower the temperature of the anode plateau,principally in high-pressure short arc lamps, by up to 200 K bycomparison with uncoated anodes.

The present invention is suitable chiefly for mercury high-pressuredischarge lamps with a content of 1 to 60 mg/cm³ Hg. A typical coldfilling pressure of the added inert gas is from 0.2 to 5 bar. Xenon isused, in particular, but argon (250 mbar) is also very suitable.

The present invention can also be applied to other types of lamp, inparticular to xenon high-pressure discharge lamps with a cold fillingpressure of up to 20 bar. A very important field of application arehigh-pressure discharge lamps which are operated in a pulse fashion orwith direct current. The point is that the loading of the electrode isparticularly high here. To date, the anode plateau has melted in themiddle and exhibited an extensive change in structure. This a problemhas now been eliminated. In principle, the technique described here issuitable not only for the anodes of this highly loaded lamp, but alsofor its cathodes. The front region of the cathode is advantageouslypointed.

FIGURES

The invention is to be explained in more detail below with the aid of aplurality of exemplary embodiments. In the drawing:

FIG. 1 shows a mercury high-pressure discharge lamp,

FIG. 2 shows the coated anode of the lamp from FIG. 1,

FIG. 3 shows a comparison of the anode temperature of two lamps,

FIG. 4 shows a further exemplary embodiment of a coated cathode,

FIG. 5 shows a further exemplary embodiment of a coated cathode,

FIG. 6 shows a comparison between the reduction in radiation of twolamps in constant operation,

FIG. 7 shows a comparison between the reduction in radiation of twolamps in pulsed operation, and

FIG. 8 shows the pulse shape for operation of the lamps in FIG. 7.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of a mercury high-pressure discharge lamp 1 witha discharge vessel 2, two shaft sections 3 and caps 4 respectivelyfastened thereon. The lamp is operated with a power of 1000 W usingdirect current (but alternating current is also possible) . The anode 5and the cathode 6 are spaced apart by 4.5 mm. The discharge vessel 2 ismade from quartz glass with a wall thickness of approximately 2.8 mm.The bulb is filled with 4.5 mg/cm³ mercury and xenon with a cold fillingpressure of 1.4 bar. The operating temperature of the bulb reachesvalues of up to 750° C. outside.

FIG. 2 shows the anode 5 in detail. The body of the anode 5 is seated inthe shape of a solid cylinder on a holding rod 10. The cylinder tapersby running together conically on the side facing the discharge. Theconical region 11 ends with a flat plateau 12 whose diameter isapproximately 30% of the cylinder. The conical region extends over aheight of approximately 6 mm. Except for its front part, whichessentially comprises the conical region and the plateau, the cylinderis coated with dendritic rhenium (13). Characteristic of this areneedle-shaped rhenium crystallites whose mutual spacing is approximately10 to 30 μm. The layer thickness is approximately 25 μm. Theneedle-shaped crystallites are positioned virtually perpendicularly onthe surface, with the result that incident radiation is virtuallycompletely absorbed by multiple reflection between the lateral surfacesof neighbouring crystallites. As a result, such a layer has a highabsorptivity and is black. According to Kirchhoff's radiation law, thehigh absorptivity is associated with a high emissivity. The emissioncoefficient of the black rhenium is approximately ε=0.9. It is importantfor the function that, on the one hand, the crystallites are positioneddensely enough, and on the other hand that the valley between thecrystallites is deep enough. The ratio between the spacing and height ofthe individual dendrites should preferably be at least 0.3.

In another exemplary embodiment of lower power (below 1000 W), theentire anode is coated with rhenium.

FIG. 3 shows a comparison of the temperature in the region of the anodeplateau and of the cylinder for a mercury short-arc lamp with a power of3500 W between an anode coated with dendritic rhenium and an anodecoated with TaC. The anode plateau is the part of the anode moststrongly loaded thermally, from which the tungsten leading to bulbblackening evaporates. The temperature difference between the twoversions is approximately 170° C. on the plateau. This difference ismaintained over the entire front region of the anode (up to a spacing ofat least 3 mm from the plateau).

FIGS. 4 and 5 show further exemplary embodiments of a highly loadedcathode 15 for a lamp of high power (3500 W) which is covered completelywith a dendritic rhenium layer 16, as well as of a less severely loadedcathode 20 for a lamp with a power of 2500 W, in which only thecylindrical body 21 is coated with rhenium 22.

FIG. 6 shows the comparison of the decrease in radiation of a mercuryshort-arc lamp whose anode is coated with dendritic rhenium (curve a) inrelation to a lamp of identical design whose anode is coated with TaC(curve b). The lamp was operated in each case at a constant power of3400 W. The total radiation intensity was measured in the wavelengthregion of 363 to 367 nm (corresponding to the i-line particularlyimportant for wafer steppers) using an Ulbricht sphere. It is clearly tobe seen that the lamp with a dendritic anode coating has a substantiallylower decrease in radiation due to blackening over the service life thanthe lamp with a TaC coating on the anode.

FIG. 7 shows the comparison of the decrease in radiation (once again inthe region of the i-line) of a mercury short-arc lamp whose anode iscoated with dendritic rhenium (curve a) in relation to a lamp ofidentical design (curve b) whose anode is coated with TaC, in pulsedoperation. This is understood as an operation in which the power isvaried periodically between at least two values, the electrodes beingextremely heavily loaded. In the exemplary embodiment shown, the mercuryvapour lamp is operated respectively for 300 ms at 2720 W and for 500 msat 2400 W. The sequence of the pulses is represented diagrammatically inFIG. 8. The power is varied in each case linearly at 1600 W/s. Atrapezoidally pulsed power signal results. With this power operation,the blackening of the lamps is stronger than in operation at constantpower. Here, as well, the lamp with a dendritic coating of the anodeexhibits a plainly better service life response (weaker blackening) thanthe lamp with TaC coating on the anode.

What is claimed is:
 1. Electrode for a high-pressure discharge lampcomprising: a core material, at least a part of the surface of said corematerial being covered with a dendritic layer made fromhigh-melting-point metal and wherein the emission coefficient of saidsurface is greater than 0.6.
 2. Electrode according to claim 1,characterized in that the high-melting-point metal is rhenium, tungsten,molybdenum or tantalum or a mixture or a chemical compound of thesemetals or of the metals hafnium or zirconium, in particular a nitride orcarbide.
 3. Electrode according to claim 1, characterized in that thedendritic layer is between 10 and 40 μm thick.
 4. Electrode according toclaim 1, characterized in that the electrode is in particular a pointedcathode which is covered at least partially with the dendritic layer. 5.Electrode according to claim 1, characterized in that the electrode isan anode which is covered with the dendritic layer completely or atleast partially, specifically in the region averted from the discharge.6. Electrode according to claim 5, characteried in that the anode has acylindrical body whose front region tapers.
 7. Electrode according toclaim 6, characterized in that the front region tapers conically and hasa flat plateau transverse to the axis of the cylinder.
 8. High-pressuredischarge lamp with an electrode according to one of the precedingclaims, said lamp having a fill containing mercury and/or inert gas. 9.Method for producing an electrode according to claim 1, characterized inthat the deposition of the layer is performed either by means of CVD orby means of PVD.